It helps a lot if you report one of the threads and in your message you provide the moderators with the URLs of any other threads you think should be combined. It doesn't matter which one you report.
Usually we'll merge the newer threads into the oldest one to keep the original thread title (and original thread number for those you have subscribed to the thread). Sometimes we decide not to merge for one reason or another, and sometimes we merge into a newer thread because is has more posts (more likely to have subscribers) or the thread title is more suitable.
In all cases the posts in a merged thread will be in chronological order.